Pinnace Studio is a popular video editing software and produces .stx files to when saving work in progress as projects. Pinnacle Studio supports export and saving of projects to several other multimedia formats, including MPEG-4 (.mp4). However, it is likely that you will need the original multimedia data as well.
As far as we know, only Pinnacle Studio can handle .stx projects and thus is your only choice for this conversion.
Updated: December 15, 2016